As a power supply that outputs direct current (DC) voltage, a DC/DC converter including a full-bridge circuit and a transformer is widely used. The full-bridge circuit includes two pairs of switching elements, each pair including two switching elements connected in series. In the full-bridge circuit illustrated in FIG. 36, a first switching element pair includes an upper arm element Sa and a lower arm element Sb, and a second switching element pair includes an upper arm element Sc and a lower arm element Sd. The upper arm element Sa and the lower arm element Sb operate in a complimentary manner (one turns off in a case that the other turns on), and the upper arm element Sc and the lower arm element Sd also operate in a complimentary manner. In a typical full-bridge circuit, the upper arm element Sa and the lower arm element Sd turn on and off at the same timing, and the lower arm element Sb and the upper arm element Sc also turn on and off at the same timing.
In a full-bridge circuit, it is necessary to reduce the loss in the switching elements. As a method for reducing the loss in the switching elements included in the full-bridge circuit, a phase-shift method of providing a phase difference between the operation timing of the upper arm element Sa and the operation timing of the lower arm element Sd and providing a phase difference between the operation timing of the lower arm element Sb and the operation timing of the upper arm element Sc is known. In the phase-shift method, after a source-drain voltage (or emitter-collector voltage) of the switching elements is reduced to substantially zero, the switching elements are turned on and off using power stored in a reactor provided on a current path of the full-bridge circuit. Thus, zero voltage switching of the switching elements is performed, making it possible to reduce the switching loss.
The phase-shift method is, for example, described in PTL 1. In PTL 2, there is described a power supply that, in order to perform zero voltage switching of the switching elements during low output, is configured to transmit power stored in an inductance included in a secondary circuit to a primary circuit via a transformer.